Ultrasonic vibrations have been used for cleaning surfaces and workpieces in a stationary fluid system such as a tank. Cleaning results through a process known as cavitation which results in the formation of a cavity that breaks up or prevents the formation of contaminants. Cavities in ultrasonic cleaning are primarily due to bubbles which expand, contract and collapse with pressure changes, thereby loosening or removing scale and other contaminants from a surface. In theory, cavities are formed on nuclei. Nuclei may take a variety of forms including, for example, small air bubbles that already exist in the liquid, small pockets of gas in cracks of walls of a liquid-containing vessel, or dust particles or other microscopic particles in the liquid. Fortunately, nuclei necessary for cavitation exist naturally on surfaces where some change, such as the removal of dirt, is desired, and thus generally occurs where it is needed most. This is probably because perfect wetting of a dirty surface does not typically occur. Instead, air may remain trapped in cracks of walls, under scale particles, or otherwise in film partially surrounding dirt particles. The air pockets and film often serve as a source of nuclei for cavitation bubbles.
The mechanism by which ultrasonic energy causes cavitation is described in Frederick, Ultrasonic Engineering, John Wiley and Sons, Inc, 1965, the contents of which are hereby incorporated by reference. In theory, a nucleus remains quiescent until some thermal, mechanical, or chemical change occurs in the liquid that upsets the equilibrium. Such changes cause the bubble to grow or collapse. Ultrasonic waves are an example of a mechanical disturbance which consists of pressure fluctuations, positive and negative, above and below the pressure of the liquid in which the ultrasonic waves are traveling. A reduction in pressure encourages a submicroscopic bubble to grow. A pressure higher than that of the liquid will discourage bubble growth or cause the collapse of one that has started to grow. It is theorized that the sudden collapse of bubbles which have started to grow, produce large instantaneous pressure at the center of the bubble that result in cleaning of surfaces. Solid material which is chemically or mechanically bonded to the surface where cavitation occurs can therefore be removed as a result of the scrubbing action of the collapsing bubbles.
Despite their widespread use to clean surfaces and workpieces in stationary fluid systems, ultrasonic energy has apparently not been applied successfully in the prior art to clean components in moving fluid systems. The internal walls and components in moving fluid systems that conduct fluids, such as water, are also subject to a build-up of scale and contaminants due to dissolved minerals and organic materials in the fluid. Over time, this build-up may have an adverse and deteriorating effect on the efficient operation of the system.
One example of this problem is typically observed in water sterilization systems that use ultraviolet (UV) lamps as the sterilizing component. High intensity UV lamps are typically encased in relatively expensive quartz sleeves and installed in a sterilizing chamber around which the water flows. Even though many water purifying units have pre-filters that remove most of the organic and larger solid materials, dissolved organic and inorganic materials, including bacteria, dirt, and dissolved minerals frequently remain in the water. Over time, these materials precipitate and deposit onto the quartz sleeve and internal walls of the UV chamber in the form of scale. The resulting scale absorbs and blocks the UV radiation degrading the sterilization process. Additionally, the build-up of scale on the quartz sleeve allows bacteria and other micro-organisms to survive and multiply in so called shadow areas on the chamber walls and on the quartz sleeve, which are created by the blocking of UV radiation in the vicinity of denser areas of scaling on the quartz sleeve.
Continuous and efficient operation of a water sterilization system usually requires relatively costly periodic maintenance, which includes shutting the system down for cleaning and descaling. In the case of the UV water sterilizing components, the UV lamp and quartz sleeve can be removed and manually cleaned and re-installed. Often, the UV lamp and sleeve are discarded and replaced. So far, there has apparently been no efficient and cost effective method developed for reclaiming or recycling of the quartz sleeve.
U.S. Pat. No. 4,752,401 (the '401 patent) describes a water treatment system for swimming pools and potable water in which UV lamps are subjected to ultrasonic energy for loosening particles tending to cling and deposit on the lamp surface. The '401 patent, however, only generally describes the ultrasonic transducer used for creating the ultrasonic energy. It does not describe the type of transducer used, nor does it describe the precise location of the transducer within the system.
A potential drawback of the system disclosed in the '401 patent is that destructive interference and nodal points may exist due to the formation of a standing wave. Nodal points are positions of zero motion and thus, if present in a sterilization chamber, create shadow areas or cold spots where bacteria and other micro-organisms can survive and multiply. Conversely, constructive interference may occur in areas where two waves add, providing for areas of enhanced amplitude, or so called anti-nodes or hot spots. Accordingly, such a prior art system which simply incorporates a transducer somewhere in the system likely would not result in effective cleaning over the entire surface of a quartz sleeve.
Perhaps reflecting the drawbacks perceived in the prior art of using ultrasonics in water sterilization systems, alternative cleaning methods that do not use ultrasonics have been developed to clean and prevent the build up of scale on quartz sleeves. In one such system, a manually operated internal wiping system is used to literally mechanically wipe the scale off of the surface of the quartz sleeve. Among their many drawbacks, the effectiveness of these wiper systems depends on the reliability of the user to operate the system on a regular basis. Another drawback to such systems is that exposure to UV radiation tends to degrade the rubber or elastomeric wipers over time. Additionally, the internal components of the wiper systems themselves produce shadow areas, and actually become sites for the growth of bacteria and other micro-organisms, which also tends to defeat the purpose of the ultraviolet sterilization action.
It is therefore desirable to have a water sterilization system incorporating a cleaning device which is automatic and yet non-invasive. The cleaning device should preferably not include bulky mechanical parts inside the chamber that may deteriorate over time. In addition, the cleaning action should be uniform and not create undesirable shadow areas.